In the prior art, relative humidity or water activity has been measured in an industrial medium, e.g. powder or granular products flowing along ducts, by putting an adsorbant or a desorbant material in contact with the atmosphere in said substance and by measuring variations in electrical parameters that result from variations in the quantity of water adsorbed or desorbed by the substance. More precisely, it is now known how to make relative humidity measurements by determining variations in electrical resistance or variations in the dielectric constant of such a material as a function of the quantity of water it contains.
A major drawback of known solutions lies in that they all require a current or a voltage to be established, with variations in the amplitude thereof providing the looked-for relative humidity information after being processed. It is therefore necessary to make use of very high quality electronic apparatuses in order to avoid any inaccuracy resulting from variations in the amplitude of the electrical quantity that are not due to variations in the quantity of water. In practice, measuring devices of affordable cost do not provide high accuracy.
The present invention seeks to mitigate these drawbacks of the prior art and to propose a novel sensor and a novel installation enabling relative humidity and/or water activity in an atmosphere, and where applicable water content in a solid medium, to be measured extremely accurately.